In recent years, AV (Audio/Visual) devices that involve digital signals such as audio signals and video signals have spread, and there are various kinds of interfaces suggested for transmitting digital signals between such AV devices. Those interfaces include widely-known ones compliant with the IEEE (Institute of Electrical and Electronics Engineers) 1394 standards, the HDMI (High-Definition Multimedia Interface) standards (HDMI being a registered trade name), and the like.
According to the HDMI Specification, a TMDS clock synchronized with TMDS (Transition Minimized Differential Signaling) data is normally transmitted from a source device to a sink (repeater) device, and the sink (repeater) device reproduces the TMDS data in synchronization with the TMDS clock.
A repeater device normally includes an expensive, high-performance clock source so as to realize high-quality audio reproduction. According to the above described HDMI Specification, however, a clock generated by an inexpensive clock source included in the source device needs to be used in reproduction, and reproduction using a clock generated by the high-performance clock source in the repeater device cannot be performed.
Also, in an application to generate video images by receiving digital data of video/audio content from more than one source device, clocks supplied from the respective source devices are not synchronized with one another. Also, the transmission formats are generated in the source devices, and therefore, the sink device needs to have a large-sized buffer and a format conversion function. In view of this, it is difficult to realize such an application.
FIG. 10 shows an outline of specifications according to the existing HDMI Specification. An HDMI transmitter exists in a source device, generates digital data of video/audio content to be transmitted and its reference clock, and transmits the digital data and the reference clock to an HDMI receiver in a sink (repeater) device as a connection destination through TMDS channels #0, #1, and #2, and a TMDS clock channel. The TMDS clock to be transmitted is equivalent to the pixel clock of the video content.
The relationship between a TMDS clock and a clock for audio reproduction, or a CTS (Cycle Time Stamp) and an N parameter, are transmitted as an ACR (Audio Clock Regeneration) packet independently from the source device to the sink (repeater) device. As a result, an audio clock can be generated from a TMDS clock in the sink (repeater) device.
In the above described HDMI specifications, the quality of the clock for reproduction depends on the quality of the clock source such as a crystal oscillator provided in the source device. It is well known that the quality of the clock for reproduction has a very large influence on reproduction of digital video/audio content. Particularly, in audio reproduction, the quality of content reproduction relies greatly on the quality of the clock.
Therefore, particularly in repeater devices, a high-quality clock source with a reduced jitter is often provided so as to realize high-quality audio reproduction. However, in a connection according to the HDMI Specification, high-quality reproduction using the high-quality clock source provided in a repeater device cannot be performed.
Patent Document 1 suggests a method of realizing higher-quality reproduction of audio data superimposed and transmitted in the same phase by the sink (repeater) device superimposing a clock for audio reproduction on differential transmission through an HEAC channel of an HDMI. At the same time, a clock for audio data to be transmitted from the sink (repeater) device through a TMDS channel can also be transmitted by using this suggestion.